This invention relates to improved electrically conductive resins based on noble metal-filled blends of carbinol-containing polyimides and conventional liquid polyepoxides, their preparation, and their use as self-crosslinking adhesives.
It is known in the art to combine polyepoxides with polyimides or polyamide-polyimide resins or their precursors. See for example, Japan. Kokai 76 90,400 (C.A. 85 193528q) where polyimide resins with improved heat resistance were prepared by reacting maleic anhydride with diamines and epoxy resins; Japan. Kokai 77 59,633 (C.A. 87 169355b) where a polyimide solution was stirred with an epoxy resin and used as a coating for electrical insulators; Japan 75 14,488 (C.A. 83 194591a) where a polyamide-polyimide resin was blended with an epoxy resin and used as an adhesive; and U.S. Pat. No. 4,374,214 (issued Feb. 15, 1983 to F. F. Holub) where a bismaleimide, selected arylene diamines, and an organic peroxide were blended with an epoxy resin to provide curable blends useful for making composite materials and molding articles with good resistance to mechanical deformation at elevated temperatures and good adhesion to inorganic materials such as metals and ceramics.
It is also known to add an amide-imide powder, anhydride-terminated polyamic acid, or amine-terminated polyimide to epoxy compounds to give crosslinked polyimide-epoxy polymers. See, for example, the epoxidized electrostatic coating powder prepared by curing an epoxy resin by mixing the resin with a finely divided solid solution of the amide-imide powder in an organic anhydride, the crosslinking being effected by the anhydride in the powder (U.S. Pat. No. 3,458,595 issued July 29, 1969 to W. W. Ulmer); the resins prepared by crosslinking the reaction products of anhydride-terminated, polyamic acids and epoxy compounds to give resins with "superior thermal stability and electrical properties, especially dielectric values" (see Col. 1, lines 42-50 of U.S. Pat. No. 3,663,651 issued May 16, 1972 to G. R. Trout); and the tractable, heat- and solvent-resistant polymers prepared using amine-terminated polyimides copolymerized with polyepoxides (see U.S. Pat. No. 4,277,583 issued July 7, 1981 to P. A. Waitkus et al.).
It is also known to crosslink carbinol-containing polyimides with other compounds containing reactive groups such as isocyanates, epoxides, (e.g., diglycidyl ether of bis-phenol A), alkyl titanates, carboxylic acid anhydrides, carboxylic acid halides, and siloxanes (see page 15, lines 12-16 of European Patent Application No. 82401866.7 published under No. 0 077 718 on Apr. 27, 1983). It is also noted therein that it is possible to add inert constituents such as pigment dyes, organic, inorganic or metallic fillers, adhesive powders, heat- and oxidation-stabilizing additives, foaming agents, etc.
It is further known to add powdered metallic fillers, such as nickel or preferably silver or gold because of their better conductivity, to epoxy resins or polyimide resins to provide adhesives, coatings, moldings and the like which are electrically conductive. However, good conductivity is not always achieved even with high percent metal loading. See, for example, U.S. Pat. No. 4,210,704 (issued July 1, 1980 to E. A. Chandross et al.), where it is disclosed that a conductivity enhancer is used in silver-filled epoxy resin adhesive formulations to increase conductivity and lower resistivity.
Metal-filled polyimides are preferred for most applications. There is therefore a need for metal-filled polyimides of improved conductivity.